studystudy 在求vip时，先用pls函数做回归，但是出现了如下警告：In pls(x, y) : Zero- or near-zero variance predictors. Reset predictors matrix to not near-zero variance predictors. See $nzv for problematic predictors.再求vip时这些Zero- or near-zero variance predictors变量的vip求不出来。
